Dancing all over the world: BBC's golden goose still flying high, but for how long?

Tonight, up to 10 million people are expected to glue themselves to BBC1 for the final of Strictly Come Dancing, the nation's nicest TV show, which has effortlessly shimmied its way around the world to become the BBC's most successful global export and most important cash cow. But after a controversial year during which the abrupt exit of cuddly political journalist John Sergeant and a snafu with the phone votes soured the programme for some, the BBC's ever-present critics wonder if lasting damage has been done.

As model Lisa Snowdon, actor Tom Chambers and bookmakers' favourite Rachel Stevens strive to become the show's sixth winner, BBC executives will anxiously hope the finale will eclipse rather than fuel the controversy.

It is impossible to overstate the importance of Strictly to the BBC and its global ambitions. Internal figures obtained by the Guardian show the amount of money BBC Worldwide, the corporation's commercial arm, is making from the show has soared in recent years. BBC Worldwide's content and production arm generated revenues of £68.5m in the year to 31 March 2008, up from £52.9m the previous year and £36.1m in 2006. An estimated 50% of that money came from a single programme: Strictly.

It has spawned spinoffs in more than 30 countries, making nearly £80m for the corporation, according to some estimates; income that is ploughed back into programme-making. But beyond the money is the sheer scale of the show's cultural impact. It is a success in countries as diverse as New Zealand, Estonia, Bulgaria, India, China and Israel.

In Denmark the wife of the prime minister was a contestant, while in Austria the show screened a special episode, Dancing Stars - the Dream Wedding, in which two professional dancers got married live on air. And in a case of life imitating art, dancer Paul Mercurio, who shot to fame as the lead in the film Strictly Ballroom, served as a judge on the Australian and New Zealand versions. Also in the Australian show, rightwing politician Pauline Hanson waltzed her way into the final while at the same time campaigning in the federal election for a seat in Australia's upper house. She lost both contests.

Wayne Garvie, BBC Worldwide's director of content and production, who commissioned Strictly in his previous job as the BBC's head of entertainment, describes it as a "slam-dunk success", a show that has solved BBC1's "Saturday night problem" for good.

The path to this year's final has not been smooth, however, disrupted by the unscheduled departure of Sergeant and overshadowed by yet another BBC voting scandal. The corporation's critics claim the way executives have handled the controversies threaten the future of the show, which has lost this year's ratings battle with The X-Factor. After unwittingly unearthing TV gold, the BBC could be in danger of tarnishing the show's global appeal.

Along with other shows identified as BBC "superbrands", including Top Gear and Doctor Who, Strictly has been used as a battering ram by executives determined to force their way into foreign markets as they attempt to top up the £3.4bn a year licence fee.

But the very international popularity has taken its toll on the judges who are integral to its success, two of whom also appear on the US version of the show. Dancing With The Stars, and their growing fame has attracted the attention of rival broadcasters. The X-Factor executive producer, Simon Cowell, has publicly professed his admiration for gregarious Italian Bruno Tonioli, while former Hot Gossip dancer Arlene Phillips has taken on other projects, devising Britannia High, a primetime drama for ITV.

The workload of Tonioli and head judge Len Goodman, who appear on the British and American versions of the show, has prompted speculation, strongly denied, that Goodman could quit - constant transatlantic travelling has already forced him to miss one US show. "I've got to be careful what I say," Garvie says. "Len and Bruno are brilliant and they are very important to what we do in the US and for the UK show." Losing either of them, he adds, "would be a major headache". But Garvie believes the "format is strong enough to survive" high-profile departures.

BBC executives will seize on tonight's viewing figures as a triumph, but the furore over Sergeant's exit, and the voting mix-up which prompted 1,800 complaints to the BBC suggests public affection for the show may be wearing thin. Sergeant, labelled a "dancing pig" by newspapers, luxuriated in his underdog status and defied attempts to oust him by appealing directly to the public, who delighted in using their votes to keep him in.

His decision to leave, remarking that winning the competition would be "a joke too far", prompted outrage from fans, many of whom felt he had been forced out by Strictly's acerbic panellists. Last week there was an outcry when all three remaining couples were allowed into the final and viewers who had voted in vain had to be offered refunds.

Garvie will not be drawn into the row, but he insists he is "incredibly confident" about its future. He says the question of the show's overseas longevity, particularly in America, "is one I ask myself every day. If it plays twice a year in the US, is it going to burn?"

The BBC has lavished attention on overseas versions of the show, although it denies it has done so at the expense of the original UK programme. "Our job as brand guardians is to ensure they get the attention they deserve," Garvie says, adding that he routinely encourages broadcasters to give it "a lick of paint".

Despite its success, the British Strictly may require a more radical makeover before it returns to TV screens next year. But no one will be bold enough to write it off for a while yet. After all, its original taffeta and satin version, Come Dancing, was with us for 50 years.

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